There has been proposed an HEMT device in the related art. The HEMT has a heterojunction structure formed by stacking a low temperature buffer layer made of GaN, a buffer layer made of GaN, an electron transit layer made of GaN, and an electron supply layer made of AlGaN, in this order, on a substrate. In addition, the HEMT includes a source electrode, a gate electrode and a drain electrode formed on the electron supply layer.
In the HEMT, the electron supply layer has a larger band gap energy than that of the electron transit layer, and a two-dimensional electron gas layer is formed below a heterojunction interface between these two layers. The two-dimensional electron gas layer is used as a carrier. In other words, when the source electrode and the drain electrode are operated, electrons supplied into the electron transit layer travel through the two-dimensional electron gas layer at a high speed and move to the drain electrode. At this time, by varying the thickness of a depletion layer under the gate electrode by controlling a voltage applied to the gate electrode, electrons moving from the source electrode to the drain electrode (i.e., a drain current) can be controlled.
In the HEMT as described above, in order to reduce a substrate-drain capacitance, the substrate may be coupled to a drain potential. However, the simple change of a coupling-targeted potential of the substrate to the drain potential may lower a dielectric breakdown voltage.